Latent image composite master and method

ABSTRACT

In the disclosed method for making a composite master capable of producing both visible and invisible images on a copy or receptor sheet, a first and second set of infrared absorptive images are entered on the obverse, uncoated surface of the master and mirror image deposits of dithiooxamide (DTO) or a derivative thereof corresponding to the second set of images are entered on the reverse surface of the master. The reverse surface is coated with a dye precursor such as a gallate. A wax transfer sheet or the like is used for providing the DTO deposits. The resulting master is capable of providing both visible and invisible entries on a nickel or copper salt-coated receptor sheet by passing the master and receptor through an infrared copying machine.

United States Patent 1191 Bildusas et al. Jan. 8, 1974 [54] LATENT IMAGE COMPOSITE MASTER 3,262,386 7/1966 Gordon 117 362 AND METHOD 3,632,364 1/1972 Thomas ll7/1.7 [75] inventors if g n Primary Examiner-William D. Martin fz s 653 1 age 0 Assistant ExaminerM. Sofocleous Attorney-Kinney, Alexander, Sell, Steldt & Delahunt [73] Assignee: Minnesota Mining and xiafirfacturmg Company, St. Paul, ABSTRACT i In the disclosed method for making a composite mas- [22] Flled' May 1971 ter capable of producing both visible and invisible im- [21 1 Appl. N0.: 140,925 ages on a copy or receptor sheet, a first and second set of infrared absorptive images are entered on the ob- 1 verse, uncoated surface of the master and mirror [52] Cl image deposits of dithiooxamide (DTO) or a deriva- 511 Int. Cl. B4lin 5/18 live there responding the of images [58] Field of Search 117/17 36.2 36% are entered on the reverse surface of the master. The 101/426 469 470 47,3, 250/65 J f reverse surface is coated with a dye precursor such as 28376 a gallate. A wax transfer sheet or the like 1s used for I providing the DTO deposits. The resulting master is [56] References Cited capable of providing both visible and invisible entries on a nickel or copper salt-coated receptor sheet by UNITED STATES PATENTS passing the master and receptor through an infrared 3,47l,309 lU/l969 Thompson 250/65 T copying machine 3.4461117 5/1969 Stricklin 250/65 T 3,481,760 12/1969 Clark et a1 117/362 1 7 Claims, 2 Drawing Figures LATENT IMAGE COMPOSITE MASTER AND METHOD FIELD OF THE INVENTION This invention relates to latent imaging techniques and paper-based feedback systems. An aspect of this invention permits simultaneous latent invisible and visible imaging of a receptor sheet. This simultaneous imaging permits the receptor sheet to be used as a test form, work sheet, or the like wherein invisible answers or other data are in register with visible questions. A further aspect of this invention relates to a composite master sheet and a method for providing it. The composite master, placed on top ofa nickel or copper saltcoated receptor sheet will transfer both visible and invisible entries onto the receptor sheet in single pass through an infrared copying machine.

DESCRIPTION OF THE PRIOR ART Composite test forms and work sheets containing both visible and invisible entries have been developed using, for example, spirit duplicating technology. These test forms are an important part of an art known as the art of paper-based feedback systems." A typical disclosure describing alatent/visible imaged copy sheet and a method of making it is contained in U.S. Pat. No. 3,451,143 (Thomas et al.), issued June 24, 1969. The Thomas et a] patent suggests that the invisible concealed image can be deposited alone or in combination with visible images from the same or from different spirit masters. The invisible image can be developed by chemical means to provide feedback. An obvious advantage of depositing the visible and invisible images from the same master is that the visible and invisible entries will be in proper register a result that is difficult to achieve using two separate masters. The obvious method for providing the combination or composite master, suggested by Thomas et al(see column 4, line 37st seq. of this reference), is to (l image the master with a heat-transferable or spirit-transferable visible dye for the visible entry portion, and (2) image the master with a dye precursor (e.g. one of the chemicals designated Material A" by Thomas et al at column 2, line 53 et seq.) to provide the invisible entry portion or feedback.

The difficulty with this obvious method is the likelihood that it will be difficult to match the strength and clarity of the visible entry on the copy sheet with the invisible entry, when the invisible entry is developed by treating the copy sheet with, for example, one of the compounds designated Material B by Thomas et al.

Good balance between the visible and invisible entries is necessary to ensure total absence of clues'to the con-.

tent of the invisible entry before its development, but good legibility of the visible entries and the developed invisible entries. Furthermore, it is desirable that the formation and use of the composite master should involve thermographic duplicating rather than spirit du- Accordingly, this invention seeks to provide a method and a means for utilizing thermographic technology to provide copy sheets containing both visible and invisible entries. This invention contemplates providing a master capable of imaging a copy sheet with both visible and invisible entries during a singlepass through an infrared copying machine. This invention also contemplates utilizing thermographic technology to equalize the strength of the visible and invisible imaging capabilities of the composite master. This invention further contemplates a paper-based feedback system comprising copy sheets wherein visible clues to the feedback are totally absent, where the feedback may be a complex text rather than a color or simple indicia means, and wherein the strength and clarity of the image that will be observed when the feedback is developed can be evaluated before development by observing the visible entries on the copy sheet.

BRIEF DESCRIPTION OF THE INVENTION It has now been found that it is unnecessary to use a visible dye to make acomposite master if two chemi- 'cally distinct, substantially colorless dye precursors are used: an iron-complexing compound (e.g. a gallate) for invisible or feedback information and dithiooxamide (DTO) or a dithiooxamide derivative for the visible information. A composite master of this invention is, initially, a typical gallate-coated master sheet. Briefly, this invention involves, (1) providing a first set of infraredabsorptive images on the obverse (uncoated) surface of the master, (2) providing a second set of infrared absorptive images on .this obverse surface and mirror image deposits of dithiooxamide (DTO) or a derivative thereof, corresponding to this second set of images, on the coated (reverse) surface of the master, (3) justaposing the resulting composite master with a nickel or copper salt-coated receptor sheet (or, less preferably, a receptor sheet coated with a cobalt or cadmium salt) such that the coated surface of the master is next to the coated surface of the copy sheet, and (4) exposing the thus-imaged master to an infrared source such that infrared radiation is directly absorbed by the obverse surface of the composite master. The result is that the first set of infrared absorptive images will induce the formation of latent or invisible entries on the copy or receptor sheet, while the second set of images induces transfer of portions of the DTO deposits to provide visible entries. The invisible entries will comprise deposits of the gallate (or whatever equivalent ironcomplexing compound is used) and the visible entries will comprise a DTO/nickel or DTO/copper colored complex (or a Ni or Cu complex of a suitable DTO derivative). Gallate orfother iron-complexing deposits can be present in the visible entries also, and this in no way interferes with the objects of this invention. The gallate-containing invisible entries. are developed with an iron salt substantially in the manner taught by the aforementioned Thomas et al. patent, i.e. by the application of a marking fluid or material containing a ferric salt. Surprisingly, it is preferred that only DTO or a derivative thereof be used to make the visible entries in this system and that only gallates or similar ironcomplexing polyhydroxy aromatic compounds be used for the invisible portion. In the most preferred practice of this invention, the gallate (or the like) and the DTO derivative are selected such that, on the copy sheet, the strength and clarity of the invisible entries, upon developmentQis the same as that of the visible entries. In short, according to this preferred practice, if the visible print on the copy sheet is clear and legible, the invisible print will also be clear and legible when developed. Typically, this preferred practice involves using methyl gallate as the iron-complexing compound and N,N-dibenzyldithiooxamide (DBDTO) as the DTO derivative.

DESCRIPTION OF THE DRAWINGS This invention can be more clearly understood with reference to the drawing, wherein FIG. 1 is a perspective view of a multi-sheet manifold used to make composite masters according to this invention.

FIG. 2 is a greatly enlarged sectional view taken along line 2 2, showing a composite master sheet of this invention juxtaposed with a receptor or copy sheet to provide visible and invisible entries on the copy sheet.

DETAILED DESCRIPTION Turning now to the drawing, it will be seen from FIG.

1 that the composite master is initially in the form of a multi-sheet manifold with perforations or the like to permit easy and convenient separation of the master sheet 11 and the transfer sheet 13. For shipping and storage, a protective sheet (not shown) is interposed between master sheet 11 and transfer sheet 13. This protective sheet can be an ordinary piece of glassine paper or the like. In FIG. 1, the protective sheet is not shown because it has been removed by tearing along the perforation, leaving only a stub 15. The master sheet 11 becomes a composite master after being imaged in accordance with the teachings of this invention. The transfer sheet 13 is coated on its upper surface with a transferable layer 14 containing DTO or a derivative thereof, preferably in intimate admixture with a particulate material such as wax. The use of such wax transfer sheets for providing mirror images on the reverse (uncoated) surface ofa master is disclosed in, for example, Newman, U.S. Pat. No. 3,459,581, issued Aug. l969. As taught in the Newman patent, the mirror images deposited on the reverse surface of a master provide visible images on the surface of a coated receptor sheet when passed through an infrared copying machine. The master sheet 11 is provided with a coating manufacturing master sheet 11, layer 16 is coextensive in area with the reverse surface of sheet 11. This layer 16 contains a gallate such as a lower'alkyl gallate or a similar iron-complexing orthodihydroxy or trihydroxybenzene or -benzoic acid or ester (e.g. catechol, pyrogallol, gallic acid, etc.). It is preferred to match, as closely as possible, the vapor pressure of the DTO derivative in layer 14 with the gallate (or equivalent compound) in layer 16. Typical written material provided for visible and invisible entries is shown on the exposed, uncoated obverse surface 12 of master sheet 11. Thus, the expression 22 9 will provide a visible entry on the receptor or copy sheet in accordance with the teachings of this invention, and this visible entry will pose an arithmetic problem for the student. The answering or feedback entry corresponding to the arithmetic problem (31) is visible on surface 12 because it has been typed or written thereon. However, the copy sheet will contain this feedback entry in latent or invisible form, and only by treating the copy sheet with a developing fluid containing a ferric salt will this invisible entry become visible on the copy sheet. The box around the answer to the arithmetic problem has been drawn such that it will show up as part of the visible matter on the copy sheet, thereby indicating where the ferric salt should be applied.

A preferred method for providing the composite (i.e. visible and invisible) imaging capability of the master is as follows: The protective sheet is removed from the manifold, leaving only the master and transfer sheets as shown in FIG. 1. Exposed surface 12 of manifold 10 is typed or written upon to provide all the visible information which will later be contained on the finished copy sheet. For best results, it is preferred to use a carbon ribbon in the typewriter, or for long hand, a sharp pencil or ball point pen containing infrared absorbing ink. During this first imaging step, well-defined image deposits containing DTO, or a derivative thereof, are I transferred to the coated surface, i.e. layer 16 of the master sheet 11. Transfer sheet 13 is in place under master sheet 11 throughout this first imaging step; therefore, corrections should be made by placing any separator sheet between the master and transfer sheets 11 and 13 and erasing. The correct information can then be entered by removing the separator sheet and retyping or rewriting. After all the information that is to be visible on the copy sheet has been entered, the master sheet 11 is separated from the transfer sheet 13 in preparation for a second imaging step. In the second imaging step, information that is to be invisible on the finished copy sheet, e.g. an answer to the arithmetic problem, is typed or written on appropriate areas of surface 12, preferably in register with the line of type or writing containing the problem. Since the transfer sheet 13 has been removed, no mirror images are transferred onto layer 16 during the second imaging step; in fact, it is preferred to protect layer 16 of the master with a sheet of paper such as ordinary writing paper or, preferably, a scavenger backup sheet coated with a suitable copper or nickel salt while typing or writing on surface 12. The pressure of writing or typing on surface 12, which is transmitted to the scavenger sheet, causes an extremely small amount of the nickel or copper salt to be transferred to layer 16 of the master sheet 11. This small amount of coreactant salt is not sufficient to deactivate the mirror image deposits of the DTO compound, but is sufficient to scavenge or deactivate any contamination of the layer 16 with the DTO-type dye precursor which may have occurred while layers 14 and 16 were in contact.

Master sheet 11 is now a composite master, since it has now been provided with the visible material or problem (22 9, etc.) and with the invisible or latent entry giving the answer to the problem (3 l As an additional aid to the student, it is desirable to replace master sheet 11 on top of transfer sheet 13 and draw a box or circle around the answer. This box or circle will appear as a visible entry on the copy sheet, indicating precisely the area to be treated with a chemical pen or other means for dispensing a coreactant iron salt capable of developing the latent entry and making it visible.

In a further optional step of this invention, master sheet 11, after being made into a composite master, is further treated with an infrared copying machine to give the composite master the ability to produce more than fifty fully legible copies. In a suitable embodiment of this step, surface 12 of the composite master is exposed to a line source of infrared radiation (in a single pass through an infrared copying machine) while a sheet of glassine paper or the like is in contact with layer 16. The glassine paper absorbs little, if any, dye precursor vapor or molten material from layer 16 and assists in driving this vapor back into the body of master sheet 11.

Another preferred method for providing the compos ite imaging capability of the master is as follows: The protective sheet is removed as described previously. The master sheet is also removed, and placed against a suitable backing sheet or surface, e.g. the platen of a typewriter or an ordinary sheet ofwriting paper. All of the visible entries are then typed or written in one area, e.g. the left-hand area of surface 12. If desired, the invisible entries can be typed or written at this time in a different area, e.g. the right-hand area of surface 12. The master sheet 11 with the visible entries typed or written in this limited area of surface 12 is then juxtaposed with transfer sheet 13 such that layer 16 is in contact with layer 14; however, the juxtaposition should be selectively arranged so that only the area of layer 16 opposite the aforementioned limited area of surface 12 is in contact with layer M. This can conveniently be arranged by cutting off, for example, the right half of transfer sheet 13. The selectively juxtaposed sheets Ill and 13 are passed through a conventional infrared copying machine at a slow speed, i.e. a dark setting. This pass through the copying machine transfers portions of layer 14 onto layer 16 providing mirror images corresponding to the infrared-absorptive visible entries on the limited area of surface 112. After this vapor and/or melt transfer step has been completed, the transfer sheet 13 can be discarded, and all the entries which are to be invisible on the copy sheet can be typed or written onto the area of surface 12 reserved for them, e.g. the right half of surface 12, if they have not already been provided as described previously. This alternative method provides a composite master sheet which can be used in the same manner as the composite master sheet produced according to the previously described method.

Other methods of imaging the composite master will occur to the skilled technician. For example, careful use of separator or scavenger sheets can obviate the need to detach master sheet 11 from manifold during the master imaging steps (anattached, fully imaged composite master 11 is thus shown in FIG. 1). It is, of course, possible to provide separately packaged master and transfer sheets rather than the manifold as in FIG. 1.

The use of the composite master I1 is illustrated in FIG. 2. In the greatly enlarged cross section of composite master 11 shown in FIG. 2, it can be seen that surface 12 is a paper-like substrate with two sets of infrared-absorptive images 25 and 27 thereon. The set of images represented by image 25 is designed to produce the invisible entries on the copy sheet 30. Thus, image 25 is, for example, the answers to the arithmetic problems. Image 27 includes the arithmetic problems which are to visibly appear on copy sheet 30. The wavy lines above composite master 11 represent a line source of infrared light provided in a known manner by a conventional infrared copying machine having a tubular lamp with a linear filament, wherein the lamp is mounted within a focus reflective housing for progressive exposure ofa printed surface. Such machines are described in US. Pat. No. 2,740,895 (Miller) and are available from 3M Company under the Trademark Thermo- Fax. It can also be seen that image 27 has a mirror image 29 precisely corresponding thereto in intelligence content and precisely opposite to obverse sur face 12 in its position upon layer 1.6., i.e. on the reverse surface of composite master lll. Image 25, however, has not been provided with a corresponding mirror image. Accordingly, infrared-absorptive image 27 causes a mixture of dye precursor vapors and/or molten material from layer 116 and mirror image 29 to transfer over to copysheet 30, while image 25 causes only dye precursor vapors from layer 116 to transfer to copy sheet 30, i.e. only iron-complexing vapors.

It is preferredto maximize vapor transfer as opposed to melt transfer and to use the maximum belt speed or minimum darkness setting of the infrared copying machine in the step illustrated in FIG. 2.

Copy sheet 30 is provided with a coating 36 which is co-reactive with DTO-type dye precursors to provide colored complexes. Preferably, coating 36 contains a copper or nickel salt of a carboxylic acid. (Alternatively, copy sheet 30 could be impregnated throughout with this color-reactive or co-reactive salt.) Coating 36 forms a stable, colored complex with DTO or derivatives thereof, but does not react with iron-complexing compounds such as the gallates. Paper sheets in the nature of copy sheet 30 are commercially available. A typical commercially available sheet comprises a paper-like substrate 32 coated with nickel rosinate in a starch binder. The mixture of melt and/or vapor trans ferable dye precursors from layer l6 and mirror image 29 which condense on coating 36 provide visible image or entry 37. The dye precursor in layer 16 does not react with coating 36 to provide a visible colored complex, but the DTO-type compound in mirror image 29 does. Since the dye precursor in layer 16 does not react with coating 36, image 25 induces the formation of a latent image or invisible entry 35 on coating 36. This invisible entry comprises an iron-complexing compound condensed on coating 36 in a pattern corresponding exactly to the pattern of image 25. Upon treatment with a ferric salt dissolved or otherwise admixed with a suitable reaction medium, invisible entry 35 will be developed and become visible. Invisible entry 35 can be developed in this manner by any suitable means such as a fiber-tipped pen provided with a reservoir containing a liquid similar to the ferric salt solution. For example, the developing fluid or marking material can be an iron salt of an aliphatic carboxylic acid dissolved in a solvent such as an alcohol.

As will be apparent to the skilled technician, the steps of the method of this invention can be varied in their sequence so long as the imaging of the copy sheet can be carried out as shown in FIG. 2. It is also possible to provide a master sheet llll wherein layer 16 is not coextensive in area with the reverse surface of the master, e.g. by piecing together portions of a gallate-coated master (for the invisible entries) and a master coated with DTO or one of its dirivatives (for the visible entries). This piecemeal approach involves extensive cutting and/or taping or pasting and is less convenient than the preferred methods.

As pointed out previously, the dye precursor in layer 16 is a polyhydroxy aromatic iron-complexing compound. The preferred iron-complexing compounds contain an orthodihydroxy benzene or orthotrihydroxy benzene nucleus substituted with an acyl, carboxyl, or esterified carboxyl radical. The preferred polyhydroxy benzene compounds are the lower alkyl C gallates, e.g. methyl or propyl gallate. The layer 14 on transfer sheet 13 preferably contains a N,N'- diorgano substituted derivative of DTO. It is preferred that the organo radical substituted on the nitrogens of the DTO have the formula RCH wherein R is aliphatic (preferably alkyl), cycloaliphatic (preferably cyclohexyl), or aromatic, e.g. phenyl. A particularly useful N,N-disubstituted DTO compound from the standpoint of its color-forming ability and vapor pressure is N,N-dibenzyl-DTO (DBDTO). The DTO derivatives with exceptionally low vapor pressure, such as N,N'-bis-(2-octanoyloxyethyl) dithiooxamide are not preferred, since it is difficult to match their low vapor pressure characteristics with the lower alkyl gallates, and it is also difficult to obtain good vapor transfer of these compounds in conventional thermographic copying machines. The compound dithiooxamide itself is fully operative in this invention, but tends to have less storage ability and less desirable color-forming characteristics than DBDTO. It is preferred that layer M on the transfer sheet 13 comprise the DTO compound intimately admixed with a particulate material such as a wax. Suitable waxes include paraffin wax, carnauba wax, bees wax, castor wax, polyoxyethylene glycols and esters or ethers thereof, candellila wax, N,N-ethylenebis-l2-hydroxy-stearamide, and mixtures thereof. In short, paraffin, ester-type, and synthetic polymeric waxes can be used, as can waxy materials such as cetyl alcohol. It is also preferred to include a minor amount ofa film-forming compound such as ethyl cellulose and a plasticizer such as tributyl phosphate, triph enyl phosphate, dicyclohexyl phthalate, a hydrocarbon oil, or the like.

A bright-colored dye can be included in layer 14 of transfer sheet 13 to enable those using this invention to see if the mirror images are being properly formed on the reverse surface of master sheet 11 during the visible imaging step.

As pointed out previously, the preferred co-reactant for the DTO-type dye precursor, i.e. the metal salt present in layer 36, is a copper or nickel salt. The preferred copper or nickel salts are soaps derived from carboxylic acids such as stearic or oleic acid, or the rosin acids. If the aforementioned scavenger sheet is used, these same salts are used to scavenge DTO contamination of the master. Nickel rosinate/clay (e.g. 40:60) is a preferred scavenger sheet coating. As to the coating on the scavenger sheet, and as to layers 14 and 16, conventional coating weights commonly used in the art are suitable. Cobalt and cadmium salts are less preferred. Silver salts (e.g. silver behenate, silver stearate, etc.) cannot be used in the copy sheets, since silver ions will co-react with both the iron-complexing compounds and the DTO-type dye precursors. However, a sheet coated with a silver salt is very useful as a proof sheet. By passing the proof sheet through the infrared copying machine with the composite master, a visual means for evaluating the definition and darkness of both visible and invisible entries is obtained as a proof. If the proof run is satisfactory, fifty or more subsequent copies can then be run off as shown in FIG. 2 with the assurance that good copies are being obtained.

Although the use of this invention has been described primarily with reference to programmed education, other uses will occur to the skilled technician.

What is claimed is:

1. A composite master for providing images on a copy sheet comprising:

an obverse surface having, in a first obverse position on said obverse surface, a first infrared absorptive image, and, in a second obverse position on said obverse surface, a second infrared-absorptive image,

a reverse surface having a layer thereon, said layer covering at least an area of said reverse surface 0pposite to said first obverse position, said layer comprising a polyhydroxy aromatic iron-complexing compound, said reverse surface having deposited thereon, in a position opposite to said second obverse position, a mirror image of said second infrared-absorptive image, said mirror image comprising a dye precursor selected from dithiooxamide and a derivative thereof, the area of said reverse surface opposite said first obverse position being substantially free of said dye precursor.

2. A master according to claim 1 wherein said layer comprising said polyhydroxy aromatic iron-complexing compound is coextensive in area with said reverse surface and wherein said mirror image is deposited on said layer.

3. A master according to claim 1 wherein said polyhydroxy aromatic iron-complexing compound is a lower alkyl gallate and said dye precursor is an N,N'-diorgano-substituted dithio-oxamide wherein at least one organo radical has the formula RCH R being selected from the group consisting of an aliphatic, a cycloaliphatic, and an aromatic radical.

4. A master according to claim 3 wherein said R is phenyl.

5. A master according to claim 4 wherein said lower alkyl gallate is methyl gallate.

6. A method of forming visible and latent images on a copy sheet comprising a. providing a composite master sheet having on the obverse surface in a first obverse position a first infrared absorptive image and a second infrared absorptive image in a second obverse position, and having on the reverse surfacea layer covering at least an area of said reverse surface opposite said first infrared absorptive image, said layer comprising a polyhydroxy aromatic iron-complexing compound, and having deposited on said reverse surface opposite said second infrared absorptive image a mirror image of said second infrared absorptive image, said mirror image comprising a dye precursor selected from dithiooxamide and a derivative thereof, the area of said reverse surface opposite said first infrared absorptive image being substantially free of said dye precursor,

b. placing the reverse surface of said master sheet in contact with a copy sheet, said copy sheet containing a metal salt color reactive with the dye precursor on the master sheet, and

c. exposing the master sheet to infrared radiation whereby there is produced on said copy sheet a visible image corresponding to said second infrared absorptive image and a latent image corresponding to said first infrared absorptive image.

7. The method according to claim 6 wherein said latent image is subsequently rendered visible by means of treatment with an iron salt. 

2. A master according to claim 1 wherein said layer comprising said polyhydroxy aromatic iron-complexing compound is coextensive in area with said reverse surface and wherein said mirror image is deposited on said layer.
 3. A master according to claim 1 wherein said poly-hydroxy aromatic iron-complexing compound is a lower alkyl gallate and said dye precursor is an N,N''-di-organo-substituted dithio-oxamide wherein at least one organo radical has the formula R-CH2-, R being selected from the group consisting of an aliphatic, a cycloaliphatic, and an aromatic radical.
 4. A master according to claim 3 wherein said R is phenyl.
 5. A master according to claim 4 wherein said lower alkyl gallate is methyl gallate.
 6. A method of forming visible and latent images on a copy sheet comprising a. providing a composite master sheet having on the obverse surface in a first obverse position a first infrared absorptive image and a second infrared absorptive image in a second obverse position, and having on the reverse surface a layer covering at least an area of said reverse surface opposite said first infrared absorptive image, said layer comprising a polyhydroxy aromatic iron-complexing compound, and having deposited on said reverse surface opposite said second infrared absorptive image a mirror image of said second infrared absorptive image, said mirror image comprising a dye precursor selected from dithiooxamide and a derivative thereof, the area of said reverse surface opposite said first infrared absorptive image being substantially free of said dye precursor, b. placing the reverse surface of said master sheet in contact with a copy sheet, said copy sheet containing a metal salt color reactive with the dye precursor on the master sheet, and c. exposing the master sheet to infrared radiation whereby there is produced on said copy sheet a visible image corresponding to said second infrared absorptive image and a latent image corresponding to said first infrared absorptive image.
 7. The method according to claim 6 wherein said latent image is subsequently rendered visible by means of treatment with an iron salt. 